A procedure has been devised for the iodination of lipid A. The resultant product, 125I-lipid A, retains its mitogenic activity and exhibits properties like that of untreated lipid A. In accord with what would be expected from the known mitogenic specificity of both lipid A and its parent compound lipopolysaccharide, 125I-lipid A binds to B cells but not T cells. This finding suggests that the cellular specificity resides in an early event in mitogenesis. The mitogenic and antimitogenic activity of certain nucleosides and nucleotides on spleen and thymus cells were demonstrated. There was no consistent correlation between the effect of these agents on enhancing the cellular content of cyclic AMP or cyclic GMP and on enhancing or inhibiting DNA synthesis. Inhibition of DNA synthesis by certain nucleotides was not reversible but it did not result in inhibition of protein synthesis. Enzymes predominantly found in the plasma membrane are not uniformly distributed among cells of the lymphoid series. 5'Nucleotidase and non-specific diesterase are readily observed in spleen but barely detectable in thymocytes of the same animals. These activities in spleen are not associated with the bulk of either T cells or B cells. BIBLIOGRAPHIC REFERENCES: Kern, M.: Surface immunoglobulins: Characteristics, mobility and role in immune phenomena. In Jamieson, G.A. and Greenwalt, P.J. (Eds.): Mammalian Cell Membranes. London, Butterworth and Co., 1977, pp. 98-113. Zimmerman, D.H. and Kern, M.: A simple procedure for labeling the lipid A moiety of lipopolysaccharide with 125I. Anal. Biochem. 71: 340-345, 1977.